Convert Uppercase to Lowercase and Vice Versa in a String

Each character has exactly one of three case-conversion outcomes

Take the string "LeArnYard 7!". Each of its 12 characters is handled on its own. An uppercase letter becomes lowercase, a lowercase letter becomes uppercase, and every nonletter stays unchanged. The result is "lEaRNyARD 7!", where the letters changed case but the space, digit, and punctuation did not move or change.

The range check must choose the direction before the offset is applied

The offset depends on the direction of the conversion. If a character is from 'A' to 'Z', add 32 to its ASCII value to reach the matching lowercase letter. If it is from 'a' to 'z', subtract 32 to reach the matching uppercase letter. A character outside both ranges must skip both operations.

'L' is in 'A'-'Z': 76 + 32 = 108, so 'L' becomes 'l'
'e' is in 'a'-'z': 101 - 32 = 69, so 'e' becomes 'E'
'7' is in neither range: no offset is applied, so '7' stays '7'

One left-to-right pass can apply the rule independently at every position

Start at index 0 and inspect one character at a time through index 11. The letters at indices 0 through 8 use one of the two range rules. At index 9 the space matches neither range, at index 10 '7' matches neither range, and at index 11 '!' matches neither range. No character must be searched for, and no character changes position.

for (char& ch : text) {
    if (ch >= 'A' && ch <= 'Z') {
        ch += 32;
    } else if (ch >= 'a' && ch <= 'z') {
        ch -= 32;
    }
}

String text = "LeArnYard 7!";
StringBuilder converted = new StringBuilder(text);

for (int i = 0; i < converted.length(); i++) {
    char ch = converted.charAt(i);
    if (ch >= 'A' && ch <= 'Z') {
        converted.setCharAt(i, (char) (ch + 32));
    } else if (ch >= 'a' && ch <= 'z') {
        converted.setCharAt(i, (char) (ch - 32));
    }
}

if (ch >= 'A' && ch <= 'Z') {
    ch += 32;
} else if (????????) {
    ch -= 32;
}

Correct guards preserve the string's length and character positions

The converted string still has length 12. Index 0 contains the converted form of the original index 0, index 1 contains the converted form of the original index 1, and so on through index 11. Case conversion changes character values only, so the space stays at index 9, '7' stays at index 10, and '!' stays at index 11.

The pass visits each of the n characters once, so its time complexity is O(n). The guarded version also avoids corrupting lowercase letters with an unconditional addition and avoids corrupting the final space, digit, and punctuation with either offset. The same three outcomes are applied independently at every position.